diff --git a/src/ol/events/condition.js b/src/ol/events/condition.js
index 427f30f2ff..a2e9e2cb63 100644
--- a/src/ol/events/condition.js
+++ b/src/ol/events/condition.js
@@ -202,12 +202,14 @@ ol.events.condition.targetNotEditable = function(mapBrowserEvent) {
 /**
  * Return `true` if the event originates from a mouse device.
  *
- * @param {ol.MapBrowserPointerEvent} mapBrowserEvent Map browser event.
+ * @param {ol.MapBrowserEvent} mapBrowserEvent Map browser event.
  * @return {boolean} True if the event originates from a mouse device.
  * @api stable
  */
 ol.events.condition.mouseOnly = function(mapBrowserEvent) {
   // see http://www.w3.org/TR/pointerevents/#widl-PointerEvent-pointerType
+  goog.asserts.assertInstanceof(mapBrowserEvent, ol.MapBrowserPointerEvent,
+      'Requires an ol.MapBrowserPointerEvent to work.');
   return mapBrowserEvent.pointerEvent.pointerType == 'mouse';
 };